This invention relates to a method of removing fluoride and sulfate ions from an aqueous stream, particularly a sodium chlorate stream. Specifically, it involves the addition of calcium chloride, a source of phosphate ion, and after removal of a first precipitate, a source of carbonate ion which forms a second precipitate.
Sodium chlorate crystals can be made by electrolyzing a sodium chloride brine stream in an electrolytic cell to produce a solution of sodium chlorate. After concentration, if necessary, the solution is cooled to precipitate the sodium chlorate crystals. In order to increase the efficiency of the crystallizer operation, it is desirable to recycle the mother liquor from the crystallizer, thereby increasing the concentration of sodium chlorate. Typically, the brine will be contaminated with sulfate and fluoride ions. Since sulfate and fluoride ions are not removed with the sodium chlorate crystals and additional sulfate and fluoride ions are constantly being added with the brine, their concentrations tend to build up in the system. It is desirable to keep the fluoride ion concentration below 1 ppm as the fluoride ion in excess of 1 ppm attacks the anode, according to the anode manufacturer. As the sulfate ion concentration builds up, the efficiency of the cell goes down and, eventually, sulfates will precipitate and clog the system. For these reasons, it is necessary, or at least very desirable, to remove sulfate and fluoride ions from the system.